Various compact power-generating devices have been developed in recent years. Fuel cells are one of the most promising systems. The advantages of fuel cells over batteries include usage of continuously replenished reactants, no moving parts and reduced thermal conversion. A particular disadvantage of fuel cells is insufficient ionic conductivity of the electrolyte. Micro-scale fuel cells are currently being investigated in an attempt to provide solutions to the latter problem.
Heat engines remain the primary choice for power conversion at many large scale power plants. Based on the power generation scale, different energy sources are used ranging from natural gas to coal to nuclear. Hydrocarbon fuels containing chemical energy are used in micro heat engines. Although the second law of thermodynamics puts a limit on the conversion efficiency of the heat engines, larger energy densities are achieved compared to lithium-ion batteries. The very first micro heat engine was developed at MIT in the 1990s. Soon after, internal combustion engines and steam engines on micro scale were developed and tested. Large viscous losses resulting from a thin boundary layer were found as one of the main disadvantage of these micro heat engines.
Thermophotovoltaics (TPV) is another class of energy conversion system. Here power generation is based on a heated emitter radiating photons which are then absorbed by a photocell and converted to electricity. This concept is very close to solar cells with one major difference—the source of radiation to power the cells. Although the emitter can be heated by sunlight, in order to provide sufficient temperature for efficient operation, extremely large beam concentrators are required which make this choice not very practical. Hence, the emitter is usually heated by combustion, providing a great deal of versatility in potential fuels. Higher power densities compared to solar cells are reported since the emitter and the photocell are in close proximity. The current challenges in TPV applications on a large scale include design, fabrication and material selection.
Solar cells are arguably the most thoroughly explored energy conversion systems. A photo-effect in semiconductors is due to generation of electrons and holes as a result of light absorption. When a semiconductor is brought in contact with an electrolyte containing a reduction-oxidation (redox) system, equilibrium is achieved by electron exchange at the interface. Illuminating a semiconductor reduces the band bending of the semiconductor and generates a photo-voltage acting as the driving force for the electron exchange.
In a typical photovoltaic (PV) module, photons of longer wavelength do not generate electron-hole pairs. That respective portion of the light energy is converted to heat. The working temperature of such a device is increased and the cell efficiency is thereby reduced. Structural damage may also occur due to overheating of the device. Thus, photovoltaic/thermal hybrid solar systems were introduced, enabling production of both electricity and heat from one integrated system. One example system is a combination of TPV and solar-assisted heat pump systems with a TPV panel directly coupled to the heat pump.
There is a general need for further alternative fuel cell developments.